Why we still don't have a cure for the common cold

It's time to move beyond cough syrup.
Modern science has eradicated smallpox, extended life expectancy, and made huge gains in battling some of the world's deadliest diseases. So why can't we knock out the humble cold?

The short answer is twofold.

First, what we think of as a cold is actually caused by many different viruses. Even the most common among those, rhinovirus, has more than a hundred different strains.

"Curing" a cold would actually mean eradicating a long list of respiratory viruses that happen to cause similar symptoms. Those symptoms, incidentally, are mostly just your immune system kicking into high gear to fight off an infection, something that can manifest as inflammation in the throat and congestion in the nose.

Second, while sniffling and coughing is no fun, a cold is pretty low down on the list of ailments that need curing. It can be a concern for infants, the elderly, or those with pre-existing respiratory conditions, but "for the majority of us, a common cold is more annoyance than threat," says Ian Mackay, a virologist at the University of Queensland.

Still, in 2002, researchers calculated that the annual cost of lost productivity because of colds is $25 billion. The National Institutes of Health estimates that people in the United States experience about one billion colds every year. What if we could make those all go away?

A couple years ago, we talked to a number of experts to get the full story on why exactly it's taking so long to knock out the common cold — and whether we ever will. In an October story for STAT, Carl Zimmer suggested that we might be inching closer toward an answer.

Unfortunately, we're not quite there yet. Here's why.

Why isn't there a cold vaccine?

Each year, multiple strains of the flu are circulating. If we can vaccinate against the most common strains of the flu, it seems as if we should be able to do the same thing for colds. But it doesn't quite work that way.

There are only about three strains of flu each season, while "there are usually 20-30 different types of rhinovirus circulating each season in one geographic area," explains Yury A. Bochkov, an associate scientist in the department of pediatrics at the University of Wisconsin School of Medicine and Public Health. Only about 10% of those will show up again the next year. That means, Bochkov says, that public health officials "cannot predict the spectrum of rhinovirus types for an upcoming cold season."

Plus, even if you could, Thomas Smith of the University of Texas Medical Branch at Galveston says, "somehow stuff 100 different strains into one shot," that would take care of only the most common cold-causing virus.

More than 200 viruses can cause what a sick person would recognize as a cold, including "some strains of influenza virus, adenoviruses, coronaviruses, enteroviruses, [and] respiratory syncytial virus," Bochkov says. A rhinovirus vaccine would do nothing to protect against those.

Still, as Zimmer notes in his report on current efforts to develop a cold vaccine, a team led by Martin Moore at Emory University developed a rhinovirus vaccine that seems to be effective in macaque monkeys. It contains 50 strains of the virus, hardly comprehensive — but the results, published in September, were encouraging nonetheless.

In addition, since colds are particularly dangerous for people with certain respiratory problems, one approach might be to test a vaccine that protects against only against the strains most likely to be risky in that group. But that wouldn't be a solution for the average person trying to dodge the sniffles going around the office.

What about a highly effective treatment?

Throat drops just don't cut it.

The main reason we don't have a treatment, Mackay says, is that the common cold is usually "a short-lived and relatively mild illness."

But trying to develop drugs to treat rhinovirus also has some particular challenges.

Smith, who worked on such research in his lab at the Donald Danforth Plant Science Center, tells us that some of the approaches they were testing "really did work," at least in the lab. (That's a long way from a clinical solution for humans.) Still, "while these compounds were pretty good at hitting a number of different strains at once, there were still a few outlier strains."

That's the tricky thing about rhinoviruses, Bochkov says: "It is difficult to find an antiviral equally efficient against 160 rhinoviruses." It's that pesky question again that makes vaccine development so difficult too: The enemy here is not one thing, but many.

Furthermore, colds are not usually life-threatening, so the Food and Drug Administration would have a very low threshold for the kind of side effects that would be considered worth it. "It really had to be nearly as safe as water for approval for the general public," Smith says. Few drugs are.

The challenges do not stop there. "Only humans show symptoms of [rhinovirus] infection," Smith says, making it nearly impossible to do any clear testing between petri dishes and human trials. Even then, researchers would first have to find a rhinovirus that test subjects had not already been exposed to — a difficult task with so many strains circulating every year.

If anyone is able to find an effective treatment, however, those efforts might pay off. "There would be a huge market among wealthy nations who have overcome some of the more serious infectious diseases and now have moved their attention to removing the annoyance of the common cold," Mackay says.

What about just trying to defeat all viruses, colds included?

Structure of the human rhinovirus capsid, the shell surrounding the virus' genetic material.

Numerous researchers are working on something called broad-spectrum antivirals, which would target a wide variety of viruses. While much of this research is still in very early phases, it may offer the best hope for an eventual cold cure — though vaccine efforts may well pan out first.

Todd Rider, formerly a senior staff scientist at MIT Lincoln Laboratory and a researcher at Draper Laboratory in Cambridge, developed one broad-spectrum antiviral compound, called DRACO.

DRACO, Rider tells us, "is designed to treat or prevent infections by a broad spectrum of viruses, just as existing antibiotics can treat or prevent infections by a broad spectrum of bacteria."

The compound has so far been effective against 15 different viruses in cells and in mice. It works by entering all cells and then destroying those in which it detects a viral infection. "For the common cold in particular, DRACO was shown in human cells to be effective against all four rhinovirus strains tested," Rider says, "and to completely eliminate rhinoviruses without harming uninfected cells."